The sprocket set structure of the conventional variable speed bicycle makes use of the conventional sprocket of the single speed bicycle, without taking into consideration the design of the chain shifting process. In other words, when the gear changing is under way, the sprocket chain engages the other sprocket in a random manner. As a result, the chance of success of gear changing of the conventional variable speed bicycle is relatively low. In the meantime, the speed changing process is slow while the operation feel is less smooth. In view of these drawbacks, various improvements in the sprocket structure of the variable speed bicycle have been made in the recent years in the hope that the engagement of the sprocket chain and the sprocket takes place successfully with precision when the bicycle gear changing is under way. The U.S. Pat. Nos. 5,085,621 and 5,087,226 disclose respectively a sprocket having the design of the slanted pocket of recess for averting the sprocket changing outer chain plate and for bracing the waist portion of the sprocket chain, so as to enhance the chance of success of gear changing. Another U.S. Pat. No. 5,192,248 discloses an invention making use of the reduction in the engaging tooth height of the deformed tooth or the affixed tooth. The fixed point gear changing is attained by means of the relay engagement of the intermediate teeth. The U.S. Pat. No. 5,413,534 discloses a gear changing path which is provided with a support projection for bracing the waist of the sprocket chain at the time when the gear changing takes place, thereby averting the chain shifting failure which is brought about by the fall of the sprocket chain before the chain has arrived at the engaging teeth.
The gear changing auxiliary structures described above have indeed greatly enhanced the rate of the gear-changing success. However, the gear-changing success is assured only when these structures and the links of the sprocket chain remain in a precise geometric relationship so as to enable the sprocket chain to complete the gear-changing action at the designed engagement position. Nevertheless, the sprocket chain is in itself an elastic and pliable transmission element such that the moving path and the precise position of each link are subject to change by the circumstance under which the sprocket chain is exerted on by the force, the contact condition and the external coordinated relationship of the sprocket. As a result, a specific geometric relationship can not be maintained between the sprocket chain links and the gear-changing auxiliary structures described above. In addition, the structure of the support sprocket plate waist is not compatible with the various sprocket chain waists made by various manufacturers. It is therefore readily apparent that the practical application of the above-mentioned gear-changing auxiliary structures is rather limited as far as the needs of the speed variable bicycle are concerned.